This invention relates to a magnetron, and more particularly to an improved construction of the end shield of a cathode structure of a magnetron.
Magnetrons are used as ultra-high frequency oscillators for use in microwave ovens or the like. Usually, in the case of the home use microwave ovens the cathode heater voltage and the anode voltage are applied simultaneously to the magnetron. Under such conditions, at the time of starting the operation of the magnetron, the normal .pi. mode oscillation would not be established until the cathode electrode is heated up to a predetermined operating temperature. Moreover, as any noticeable anode current does not flow during starting, the anode voltage is higher than during normal operation. As the thermal electron emission from the cathode electrode increases from such condition, a portion of the electrons that collide upon the end shield of the cathode structure generates secondary electrons which collide upon the surface of the other end shield confronting said first mentioned end shield on the other side of the cathode electrode thus generating another secondary electrons which creates secondary electron multiplication phenomena. A portion of the secondary electrons is attracted by the anode electrode thus passing a small anode current. However, as the thermal electron emission increases, the space charge in the operating space changes the distribution of electric field whereby the secondary electron multiplication phenomena disappear abruptly. Accordingly, the anode current also rapidly decreases to zero whereby a large counter electromotive force is created in the high voltage of the power supply thus generating a surge voltage in the form of a pulse. Then, the normal .pi. mode oscillation condition is reached after a higher mode oscillation other than the .pi. mode. In this manner, when a surge voltage is generated at the time of starting the operation of the magnetron, insulations of not only the filter circuit for the magnetron but also of the high voltage source transformer, rectifier, etc. are destroyed. This not only endangers the operator but also causes a fire hazard.
For the purpose of obviating these problems it has been proposed to weld zirconium plates to the opposing surfaces of a pair of end shields. According to this construction, the low secondary electron emission ratio of zirconium is utilized to suppress the secondary electron emission, but as the secondary electron emission ratio of the zirconium plates is influenced greatly by the surface irregularity, condition of oxidation, and matters deposited on the surface, for example evaporated cathode substance, it is difficult to stably suppress the secondary electron emission. Furthermore as the zirconium plates are liable to deform when they adsorb gas even when the plates are securely fixed in the desired positions at the beginning of the use, the plates adsorb gas and deform during use thus causing short circuits of the electrodes. To prevent such deformation, it is necessary to firmly secure the zirconium plates to the end shields. To be securely fixed, the zirconium plates must be thin. However, from the standpoint of the deformation during use and the property of adsorbing gas, the zirconium plates must not be too thin. Thus, the property of gas adsorption and prevention of deformation contradict each other so that it is difficult to simultaneously satisfy these contradicting requirements. Further, zirconium plates react vigorously with hydrogen, and once such reaction occurs, the effect of suppressing the secondary electron emission decreases greatly. For this reason, it is necessary to fabricate the cathode structure by brazing the component parts thereof in vacuum, thus requiring complicated and expensive manufacturing installations. As an alternative, it has been proposed to weld the zirconium plates on the end shield surfaces after the fabrication of the cathode structure has been completed. However, in such method it is extremely difficult to weld the zirconium plate completely. Thus, it has been difficult to obtain a construction of the cathode electrode capable of efficiently preventing generation of surge voltage and insulation puncture.